Spherical Ablation Research Articles

Radiofrequency thermal ablation: computer analysis of the size of the thermal injury created by overlapping ablations.

The purpose of this study was to perform a computer analysis of the size of the thermal injury created by overlapping multiple thermal ablation spheres. A computer-assisted design system was used to create three-dimensional models of a spherical tumor, a spherical tissue volume consisting of the tumor plus a 1-cm tumor-free margin, and individual spherical ablations. These volumes were superimposed in real-time three-dimensional space in different geometric relationships. The effect of the size and geometric configuration of the ablation spheres was analyzed with regard to the ability to ablate the required volume of tissue (tumor plus margin) without leaving untreated areas or interstices. The single-ablation model showed that if a 360-degree 1-cm tumor-free margin is included around the tumor targeted for ablation, radiofrequency ablation devices producing 3-, 4-, and 5-cm ablation spheres can be used to treat 1-, 2-, and 3-cm tumors, respectively. The six-sphere model, in which six ablation spheres are placed in orthogonal planes around the tumor, showed that the largest tumor that may be treated with a 3-cm ablation device is 1.75 cm, whereas 4- and 5-cm ablation spheres can be used to treat tumors measuring 3 and 4.25 cm, respectively. The 14- sphere model showed that addition of eight more spheres to the six-sphere model increased the treatable tumor size to 3, 4.6, or 6.3 cm, depending on the diameter of the ablation sphere used. For treating larger tumors, we found a cylindrical model to be less efficient but easier to control. Our computer analysis showed that the size of the composite thermal injury created by overlapping multiple thermal ablation spheres is surprisingly small relative to the number of ablations performed. These results emphasize the need for a methodic tumor ablation strategy.

Surgically-induced astigmatism after laser in situ keratomileusis for spherical myopia.

To characterize the surgically-induced-astigmatism (SIA) associated with spherical LASIK. Refractive outcomes in 70 eyes that underwent primary myopic LASIK with purely spherical ablation were analyzed. The Summit Apex Plus excimer laser was used. The Bausch & Lomb Hansatome with the 180-microm plate was used to produce superiorly hinged flaps. The relationship between refractive astigmatism and corneal topographic astigmatism was analyzed using linear regression and vector analysis. There was a statistically significant negative correlation (slope = -0.21) between refractive surgically-induced astigmatism and preoperative topographic cylinder. A 0.24-D with-the-rule shift was also found. Surgically-induced astigmatism was not correlated with the magnitude of laser ablation. The lamellar keratotomy portion of LASIK reduces pre-existing corneal astigmatism and produces a relative steepening of the hinge meridian.

Laser in situ keratomileusis after photorefractive keratectomy for myopic regression

Purpose: To assess the efficacy and safety of laser in situ keratomileusis (LASIK) for regression after excimer laser photorefractive keratectomy (PRK).Setting: Eye Research Center, Istanbul University; Istanbul, Turkey.Methods: Laser in situ keratomileusis to treat residual myopia ranging from, 1.50 to 12.50 diopters (D)(mean 5,96 D ± 3.06 [SD]) was performed in 45 eyes of 25 patients. Cylindrical corrections were done in 7 eyes (15.6%) and spherical ablations, in 38 (84.40. The mean interval between primary PRK and LASk retreatment was 18.50 ± 8.12 months. The procedure was performed under a hinged corneal flap using the Chiron Automated Corneal Shaper and Chiron Keracor 116 excimer laser.Results: Six months after LASIK retreatment, mean spherical equivalent refraction was -0.67 ± 0.77 D. Thirty-six eyes (80%) were within ± 1.00 D of emmetropia. Uncorrected visual acuity was 20/40 or more in 31 eyes (68.9%), 2 eyes (4%) lost 2 lines of best spectacle-corrected visual acuity. There was no statistically significant difference in corneal haze before and after LASIK.Conclusion: Laser in situ keratomileusis was safe and effective for treating residual myopia after excimer laser PRK.

Excimer laser surgery for keratoconus

Purpose: To determine whether there is increased risk associated with excimer laser surgery of primary keratoconus.Setting: Department of Ophthalmology, County Council Hospital, Ryhov, Jönköping, Sweden.Methods: Twenty-four eyes in 23 patients with keratoconus had photorefractive keratectomy (PRK) to reduce the steepness of the cone. A VISX Twenty-Twenty B laser (193 nm) was used for the treatments. Spherical ablations, cylindrical ablations, or both were used. Patients were followed for a mean of 22 months (range 6 to 46 months).Results: Fourteen patients (58%) had improved visual acuity. Eleven (46%) could manage with only spectacles, and three (13%) were fitted with contact tenses to get adequate visual function. All treated corneas healed, and no acceleration of the keratoconus was seen.Conclusion: No increased risk was associated with treating primary keratoconus with excimer laser PRK. We found that excimer laser surgery can improve vision and the ability to wear contact lenses, and it did not interfere with subsequent corneal transplantation surgery.

Regression and its mechanisms after laser in situ keratomileusis in moderate and high myopia

Objective The purpose of the study was to evaluate the degree and mechanism of regression after laser in situ keratomileusis (LASIK) on moderate to highly myopic eyes during the first postoperative year. Design A prospective, single-center, clinical trial. Participants A total of 52 eyes of 38 patients were entered in the study; 47 eyes had complete data available at each postoperative visit. Intervention The intervention was LASIK using the microkeratome to create an 8.5- to 9.0-mm diameter, 130- to 160-μm-thick flap. A spherical midstromal multizone ablation (inner zone, 4.5 mm; outer zone, 5.5–6.0 mm) was then performed using the Summit OmniMed excimer laser (Summit Technology, Inc, Waltham, MA). The mean preoperative refraction was −14.02 diopters (D). Retreatment for undercorrection and regression was performed between postoperative months 3 and 6 on 13 eyes. Main outcome measures Manifest spherical equivalent, mean central corneal power, and central corneal thickness were the parameters measured. Results At 3 months, follow-up data were available on 47 eyes. The mean refractive regression was −1.07 D (7.6%) from the first week to the third month. During the first postoperative year, the mean regression of manifest spherical equivalent (MSE), increase in corneal power, and increase in corneal thickness were symmetric in magnitude and time course for the 34 eyes that did not require retreatment (−0.96 D, +1.03 D, and 15 μm, respectively). Conclusion Early regression of refractive effect after LASIK appears to be a consequence of an increase in corneal thickness associated with central corneal steepening. No evidence of progressive corneal ectasia was observed during the first year of follow-up. Longer follow-up is required to confirm these trends.

Photoastigmatic refractive keratectomy in 168 eyes: Six-month results

Using a 193 nm excimer laser, we performed elliptical astigmatic and spherical corneal ablation on 168 eyes with compound myopic astigmatism and analyzed the three- and six-month postoperative results. Mean preoperative astigmatism was 1.51 ± 0.81 diopters (D). Mean three- and six-month postoperative astigmatism was 0.70 ± 0.58 D and 0.67 ± 0.60 D, respectively. Six months after surgery, eyes with less than 1.00 D preoperative astigmatism improved 4.8%. The improvement was 53.5% and 59.1 % for eyes with a preoperative astigmatism of 1.00 D to 2.00 D and above 3.00 D, respectively. Overall, astigmatism improved 55.6%. There was a wide distribution from overcorrection to undercorrection. Astigmatism six months after surgery was within 10 degrees of the preoperative axis in 46.1 % of eyes.

Photorefractive Keratectomy in 472 Eyes

Between July 1990 and July 1992, 472 eyes were treated with photorefractive keratectomy (PRK) using the ExciMed UV200 193-nanometer excimer laser (Summit Technology, Inc, Waltham, Mass). The sphere equivalent refractions ranged between -1.25 and -9.60 D, with less than 1.25 D of astigmatism. Eyes with refractive error up to -6.00 D were treated with a spherical ablation, as determined by the surgeon's decision and the algorithm built into the laser software. In this group, 92% had a refraction within 1.00 D of intended correction. Eyes with more than -6.00 D of myopia were treated with a dual-zone procedure, using a nomogram developed by the author. After 6 months, 45% of eyes had a refraction within +/- 1.00 D, and 84% were within +/- 2.00 D of emmetropia. Excimer PRK up to -6.00 D is safe and predictable; above -6.00 D it is less predictable.

Quantitative Analysis of Wound Healing after Cylindrical and Spherical Excimer Laser Ablations

Superficial corneal ablation using the 193 nm excimer laser for the correction of myopia and astigmatism produces histologic changes in the corneal profile that can be quantitatively analyzed. Using the excimer laser, spherical and cylindrical corneal ablations were performed on rabbit eyes at depths of 2, 4, 6, and 8 diopters. After 12 weeks, the corneas were examined histologically using a dichlorotriazinyl aminofluorescein stain for collagen, and the results were quantified using a digital video image analysis system. Epithelial thickness and thickness of new collagen were positively correlated with the depth of ablation after spherical ablations. With astigmatic ablations, epithelial thickness and new collagen production were correlated with ablation depth in one axis only. These results suggest that during cylindrical corneal ablations with the excimer laser, radially asymmetrical stromal ablations produce quantitatively asymmetrical wound healing.

Stationary flow model of ablatively imploded spherical shells

The stationary flow model of spherical ablation is extended to shells, solutions with a density discontinuity at the critical surface, and charged-particle-beam-driven ablation. Parameter studies of the shell solutions show the relationship between shell aspect ratio, relative ablative mass removal or burnthrough, laser power, and shell material type. The discontinuous solutions are shown to occur when the critical surface and sonic surface coalesce. The relationship of these discontinuous solutions to particular physical situations is shown to be ambiguous in a way that must be resolved by microscopic transport calculations. Charged-particle-driven ablative implosion processes are shown to resemble laser-driven ablation. However, qualitatively different ablation processes occur in different regimes of the power and particle range of the incident beam. Procedures are described by which stationary solutions can be used to predict and interpret the results of experiments and numerical simulations.

Onset of nonlinear saturation for Rayleigh-Taylor growth in the presence of a full spectrum of modes.

It is generally recognized that a single Rayleigh-Taylor unstable mode grows exponentially, proportional to the initial amplitude, until the amplitude is about (1/10 to (1/5 of the wavelength. The growth then becomes nonlinear, and the mode evolves into spikes and bubbles. This paper considers how this picture of the transition to nonlinearity changes when, instead of there being a single mode, there is a full spectrum of modes. We argue that nonlinear behavior begins whenever the sum of modes over a specified small region of k space becomes comparable to the wavelength. In the case of a single mode, this reduces to the usual comparison of the mode's amplitude with its wavelength \ensuremath{\lambda}. But if the modal amplitudes are smooth functions of k, the modes begin to saturate when their amplitude is comparable to ${\ensuremath{\lambda}}^{2}$/R times a dimensionless scale factor; here, R is the radius in spherical geometry, or the length of the surface in planar geometry. Given this new notion of the amplitude at which nonlinear saturation begins, we construct a simple model to estimate the net perturbation resulting from a broadband initial spectrum. We assume that modes grow exponentially until saturation occurs, and then the growth of the individual modes becomes linear in time. The model predictions in two and three dimensions are compared with Read and Young's experiments [Atomic Weapons Research Establishment Report No. 011/83, Aldermasten, 1983 (unpublished)], and to Youngs's calculations [Physica 12D, 32 (1984)]. The experimental results are used to set the single parameter characterizing the onset of nonlinearity. The model provides a complete description of a weak dependence on initial amplitude. The model can be easily extended to any situation for which one can estimate single-mode growths; results are presented regarding effects on multimode growth of spherical geometry, ablation stabilization, and interface coupling.

Scaling laws for plasma production and pellet acceleration by laser

The results from a spherical-symmetry hydrodynamic calculation are adapted to describe the production of large plasmas by the laser ablation of solid-density pellets. Expressions are given for the rate of ablation, plasma temperature and ion energy. The upper limit on the ionized particle number is set by pellet disintegration following heating by laser-driven shock. Scaling laws are derived for the total plasma energy and particle number. Plasma conductivity transforms one-sided illumination into an approximately spherical ablation, allowing a discussion of the scaling laws for laser-induced acceleration. The reduction in laser acceleration due to quasi-spherical ablation has negative implications for laser acceleration as a method of reactor re-fuelling.

Structure and observable characteristics of laser driven ablation

Numerical hydrodynamic-heat flow simulations of spherical ablation have been done with sufficient spatial resolution to show the details of the ablation front structure. These simulations show a continuous qualitative change in the velocity spectrum of expanding ions with increasing laser pulse length, ranging from approximately isothermal behavior from short pulse to ablative behavior from longer pulses. The ablative behavior is characterized by an energetic peak in the ion spectrum, a characteristic of ablation that is experimentally observable. The longer pulse cases are found to be in good agreement with the stationary flow model of ablation.